1. Field of the Invention
The present invention relates to the field of integrated image sensor circuits.
2. Description of the Related Art
An integrated image sensor is used to convert light impinging on the sensor into electrical signals. An image sensor typically includes one or more (e.g., an array of) light-sensing photoelements such as photodiodes, phototransistors, or photodetectors, where electrical signals are generated via the well-known photoelectric effect. These signals may then be used, for example, to provide information about light intensity, color, or the optical image focused on the sensor. One common type of image sensor is a CMOS image sensor.
FIG. 1 shows a block diagram for a CMOS image sensor 100 implemented in a single integrated circuit or chip. Sensor 100 comprises a photoelement array 102, a decoding/buffer area 104, and control, processing, and input/output (I/O) circuitry 106. Photoelement array 102 comprises an array of photoelements and associated circuitry such as switches and amplifiers. Each photoelement and its associated circuitry are collectively referred to as a pixel.
Sensor 100 can be switched into one of three operating modes: normal operation, standby, and power down. A typical operating cycle for sensor 100 in the normal operation mode has three steps: acquisition or integration, read-out, and reset During the integration step, incident photons generate electrical charge that is accumulated within each photoelement in photoelement array 102 where the voltage across the photoelement is changed due to this charge accumulation. During the read-out step, the voltage is read out passively, i.e., without destroying the accumulated charge. During the reset step, the accumulated charge is drained from the photoelement thereby preparing it to receive photons during the next operating cycle. During the standby mode, if light is impinging on photoelement array 102, the photoelements are converting light to current but the charge is not accumulated. During the standby mode, the associated circuitry within photoelement array 102 and the circuitry within blocks 104 and 106 used to operate the photoelements are typically powered up with only certain biases and control signals turned off so that the sensor can be quickly switched to the normal operation mode. During the power down mode, power is completely removed from the sensor.
FIG. 2 shows a schematic circuit diagram of part of integrated CMOS image sensor 100 of FIG. 1. FIG. 2 shows a pixel 202 of photoelement array 102 of FIG. 1, a voltage source Vdd 204, a bias transistor 206, and a read-out circuit 208. Bias transistor 206 and read-out circuit 208 are typically implemented in blocks 104 and/or 106 of FIG. 1. Also shown are devices 210 that draw power from the same voltage source 204. Devices 210 may include but are not limited to on-chip circuitry, such as in blocks 104 and 106 of FIG. 1. Pixel 202 comprises a reset transistor 212, a photoelement 214, a source follower transistor 216, and a row select transistor 218.
During an integration step of the normal operation mode, reset transistor 212 and row select transistor 218 are turned off, and the voltage across photoelement 214 discharges in response to the incident light. After a predetermined integration time, transistor 218 is turned on to select the particular photoelement in the array and to initiate a read-out step of the normal operation mode. Read-out circuit 208 samples the gate voltage of transistor 216. During a reset step of the normal operation mode, reset transistor 212 is turned on, which drains the charge accumulated by photoelement 214 into Vdd 204. This charge is thus irreversibly lost. During the standby mode, transistors 212 and 218 are turned off, but power is still applied to circuitry within sensor 100 (e.g., read-out circuit 208) and possibly devices 210. During the power down mode, power supply Vdd 204 is disconnected from sensor 100.
Power consumption is important for portable and embedded imaging applications since the sensor is typically powered by a portable power source such as a battery. During the standby mode, the sensor consumes power due to off currents or other charge leakage through read-out circuit 208, devices 210, and/or other circuitry within sensor 100. This problem may become particularly serious for CMOS technology as it scales down to deep submicron levels due to increasingly higher doping densities, thinner oxide layers, and shorter channel lengths. Often, standby power consumption will limit battery life and restrict use of image sensors.
Embodiments of the present invention are directed to a technique for reducing power consumption in integrated image sensors by which at least some of the charge generated by the sensor (e.g., during the sensor""s normal operating mode) is used to power circuitry (e.g., during the sensor""s standby mode), instead of drawing the entire current from a power source such as the external battery used to power the image sensor.
According to one embodiment, the present invention is a method for operating an integrated circuit having an image sensor with at least one photoelement comprising the steps of (a) generating charge by the photoelement and (b) applying at least a first portion of the charge generated by the photoelement to other circuitry to reduce consumption of power from a power supply.
According to another embodiment, the present invention is a circuit comprising (a) an image sensor having at least one photoelement implemented in an integrated circuit and (b) other circuitry, wherein the photoelement generates charge that is applied to the other circuitry to reduce consumption of power from a power supply.